The use of ink jet printing in wide format applications is expanding. In wide-format ink-jet printing, substrates, from rigid panels or flexible roll-to-roll webs, are supported relative to an ink-jet printhead. The printhead typically prints by moving transversely, relative to the substrate at a printing station where the substrate is supported, to print a row of an image on the substrate. The printhead moves across the substrate on a bridge that extends transversely across the substrate at the printing station, carrying the printhead on a carriage that is moveable on the bridge. Such a row of the image is typically formed of a plurality of lines of dots jetted from a corresponding plurality of nozzles on the printhead. A complete image is formed by printing a plurality of such rows side by side in a scanning motion by indexing the printhead longitudinally relative to the substrate. Traditionally, there has been no relative movement between the printhead and the substrate during the transverse movement of the printhead over the substrate when printing a row of the image. Between the printing of each row of the image, however, longitudinal indexing of the substrate relative to the printhead is carried out. This indexing can be achieved by moving the substrate longitudinally on its support or by moving the bridge relative to the support. A printing system that provides both types of longitudinal movement is disclosed in U.S. Pat. No. 6,012,403, hereby expressly incorporated by reference herein.
The relative movement between the printhead and the substrate in the longitudinal direction, that is, perpendicular to the transverse row-printing movement of the printhead, requires that the indexing distance be achieved with sufficient precision to avoid visible artifacts in the printed image caused by tolerances in the lengths of the indexing steps between the printing of the transverse lines of dots of adjacent rows. The degree of precision required depends, in addition to the resolution requirements of the particular application, on the nature of the ink being jetted and the physical properties of the substrate. For example, much wide format printing is for posters, banners and signs that are printed on vinyl substrate webs, either by roll-to-roll or roll-to-sheet processes. Traditionally, these substrates have been printed with solvent-based inks that form dots that spread somewhat on the vinyl substrate before drying. Such dot spread tends to forgive longitudinal feed errors of several thousandths of an inch. This dot spread, however, limits the resolution of the image being printed and the overall quality of the image.
Advantages in wide format ink jet printing have resulted from the use of inks that are cured by exposure to ultraviolet light. These UV-curable inks can produce superior images in many applications and can print on some substrates on which other inks cannot. Furthermore, UV-curable inks do not have some of the occupational and environmental disadvantages of some other inks. Examples of ink-jet printing with UV ink are described in U.S. Pat. Nos. 6,312,123; 6,467,898; 6,523,921 and 6,702,438 and in PCT publications WO02/078958 and WO02/18148, hereby expressly incorporated by reference herein.
Advantages of UV inks over solvent-based and other inks include, for example, less dot spread, particularly on substrates such as vinyl. Such property of UV inks can provide higher resolution. Higher resolution can, however, reveal artifacts such as those caused by feed or indexing tolerances between scan rows of the printhead. The human eye, for example, can detect defects of less than 1 mil (i.e., <0.001 inch). This has created problems with roll-fed substrates, particularly smooth, low-absorbency substrates, that can occur when the dot-spread is minimal.
Web fed printers are particularly prone to longitudinal feed errors that have been difficult to control. Cumulative tolerances in the drive linkages, potential slippage of the substrate on the rollers, and other mechanical limitations have produced errors that are difficult to predict when attempting to longitudinally index a web, particularly a web of highly flexible material. Attempts to improve indexing precision between the printhead and the substrate have focused on feed controls. The use of an encoder, for example, to measure the actual feed of the substrate relative to the printhead bridge, has been attempted. The use of an encoder in a closed loop control of the substrate feed drive has been only moderately successful because of a lack of control “stiffness” in the loop. The use of an encoder to read the results of an indexing step and feed the results back to the control to make a subsequent correction has presented other problems.
When error signals from encoders have been received by feed system controllers following a longitudinal feed step, time is consumed in making a post-feed correction, delaying the transverse printhead scan. Further, the correction feed step is also prone to error, which can require a still further corrective move. In addition, the error can indicate that the substrate has been fed too far, requiring a negative correction step, or a backward move of the web. Not all machines are capable of executing reverse moves of a substrate web, and many of those that can reverse the substrate feed cannot do so accurately or efficiently. As a result, deliberately under-feeding the web has been tried. Underfeeding of the web increases the likelihood that a correction is needed and increases the overall likely number of corrections that must be made. As a result of these difficulties, high quality ink-jet printing with UV ink onto smooth substrates has not been realized in most applications where the above problems are presented.
Accordingly, there is a need for a way to increase precision in the relative longitudinal feeding between printheads and substrates, particularly smooth substrates such as vinyl, and particularly when printing with UV inks.